Metal castings are heat treated to produce a change in mechanical properties by changing the type and proportion of phases present in the solid state, the morphology of the microconstituents, and the concentration and distribution of crystal defects.
The description will be mainly in terms of copper-containing aluminum alloys, but should be understood to be more broadly applicable, where effective (e.g. some precipitation hardening systems derive strength from Mg.sub.2 Si or MgZn.sub.2 instead of CuAl.sub.2).
Such aluminum alloys (which may contain, for example, generally on the order of up to 5% copper) are currently heat treated for the purpose of improving their mechanical properties by precipitation hardening involving a solution and aging treatment sequence.
Hardening and development of other properties of aluminum-copper alloys require control of the casting and associated heat-treating processes under such conditions so as to maintain in solid solution the copper within the aluminum matrix. Following the casting mold removal, the casting typically is naturally cooled well below 470.degree. C. (often to ambient temperature) prior to the next solution heat treating step (which latter step has the purpose of re-incorporating the copper atoms into the aluminum molecular matrix, to avoid uncontrolled and excessive precipitation of copper as CuAl.sub.2 ; because copper, fully dissolved in the liquid aluminum, naturally tends to precipitate from the aluminum as the temperature decreases from about 500.degree. C. to ambient temperature).
In order to maintain the copper dissolved in the proper amount and in the required form in the alloy to obtain a predetermined level of hardness and strength, such heat-treatable aluminum castings are universally subjected to this traditional solution heat treatment at temperatures above 470.degree. C. (typically in the range between 480.degree. C. and 495.degree. C.) for a certain period of time, usually in the range between at least 2 to 7 hours. The expressed object of this heat treating step is to obtain a homogeneous distribution of fine copper precipitates in the alloy. This solution heat treating, however, incidentally adversely promotes the spheroidization of silicon and consequently somewhat degrades the machining properties of the resulting castings (a condition which the industry for most purposes has learned to accept).
The next manufacturing step is rapidly to quench said castings, without interruption, from the solution heat treatment temperature, e.g. about 480.degree. C., down to a temperature around 85.degree. C., thus maintaining the copper precipitates in the adequate amount and homogeneous distribution in solid solution. Quench cooling may commonly carried down to any of a number of different temperatures and at different rates according to the final properties of the alloy to be emphasized (see "Quenching" discussed in the ASM Handbook, Volume 4 (1991), infra at page 851 et seq. which discloses use of cold water, for near ambient temperatures; boiling water, for 100.degree. C.; polyalkaline glycol, for even higher temperatures; forced air or mist; etc.).
This quenching step produces a supersaturated solid solution that causes the alloy to harden naturally as time passes. Finally in order to accelerate and improve hardening, the castings are maintained at temperatures of about 200.degree. C. in an "aging" furnace. The time spent in the "aging" furnace brings the alloy to at least a partial coherency in its structure giving it the required hardness and strength properties.
For more information on the details of the prior art methods, reference is made to the ASM Handbook, Volume 2 (1990), entitled "Properties and Selection: Nonferrous Alloys and Special Purpose Materials"; and Volume 4 (1991), entitled "Heat Treating" (especially pages 824-879); both being tenth editions, published by ASM International; the contents of which are incorporated herein by reference. See page 833 which speaks of "the required solution heat treatment", page 844 which in discussing "Solution Heat Treating" states that "to take advantage of the precipitation-hardening reaction, it is necessary first to produce a solid solution. The process by which this is accomplished is called solution heat treating," and page 851 where the only indicated exception is discussed in "Precipitation Heat without Prior Solution Heat Treating Treatment", there stating that certain thin extrusion alloys after having been "air cooled or water quenched directly from a final hot-working operation", "develop strengths nearly equal to those obtained by adding a separate solution heat treating operation" emphases added!.
The present invention is based on the applicants' finding that by directly quenching the heat-treatable aluminum casting after demolding, contrary to the current practice of heat treating previously cooled aluminum castings in a solution furnace, the essentially same properties of hardness and strength can be obtained. Some properties may improve and others slightly decrease, but usually not more so than would occur in variations resulting from adjustments in the heat solution treatment made to emphasize one property trait over another (in the usual compromises made in such treatments to achieve the best balance of desirable properties). Even where there may be some decrease, this has been found to be within the usual tolerance levels normally required for the final product.
This invention thus results in multimillion dollar savings in capital investment and upkeep costs of the solution heating treatment furnace and the operational energy costs of such treatment. The casting plants are therefore greatly simplified. This new and simplified heat treating process thus constitutes a significant breakthrough in the art of heat-treatment aluminum alloy casting.
As an example of the heat treating step of the prior art, which is avoided by the present invention, reference is made to U.S. Pat. No. 5,294,094 to Crafton et al. In FIG. 1 of this patent the "solution furnace" is designated by the numeral 11 and as described therein. It comprises a number of zones and is the largest piece of equipment of the plant, involving high capital costs. This patent is addressed to the improvement of such heat treating furnace by performing the sand core removal therein, consequently it does not suggest the elimination of such furnace as the present invention does.
The present invention provides a process which eliminates the traditional "solution furnaces" and produces aluminum alloy castings with similar properties of hardness and strength as those of the prior art. Another advantage of the invention is that silicon spheroidization is avoided improving the machining properties of castings. The effect is that the castings produced according to the invention improve to class A from class B in the classification for aluminum alloys. Aluminum alloys are classified from A to E in increasing order of chip length and decreasing order of quality of finish. Class A is characterized as free cutting, very small broken chips and excellent finish; class B is characterized as curled or easily broken chips and good to excellent finish.
The silicon morphology in the castings is responsible for the machining properties. Silicon takes the form of plates in the naturally solidified alloy, but when the alloy is heated to the solution temperature, after it has been cooled down, then silicon changes to spheroid form which produces continuous curled chips. If the alloy is quenched and aged in accordance with the invention without the solution heating step, the silicon keeps the fibrous structure which advantageously produces short chips.
The time involved in heat-treating for solution of copper has been decreased in a factor of about 4 from the traditional 8-12 hours, e.g. to 2-3 hours according to M. H. Lavington, The Cosworth Process-a new concept in aluminum alloy casting production, Metals and Materials, Volume 2, No. 11, November 1986, but the solution treatment has not been eliminated. The applicants are not aware of any proposal from aluminum casting technology suppliers, or of any plant currently operating, which have either suggested or practiced a heat treatment process as in the present invention, i.e. without subjecting said castings to the solution heat treating furnace and instead, contrary to prior art expertise to directly quench the castings to near ambient temperature immediately after demolding, or at least after maintaining the temperature of such casting above 400.degree. C. (in other words, without letting said temperature to fall below 400.degree. C.).